As technological development of mobile instruments and demands thereof continue to increase, demand for batteries as an energy source is also increasing. Among such secondary batteries, a lithium secondary battery having high energy density and action potential, a long cycle life and reduced self discharge has been made commercially available and has entered widespread of use in the art.
An electrode assembly having a cathode/separator/anode structure to form a secondary battery may be broadly classified into a jelly-roll (winding type) assembly and a stack (laminate type) assembly, in terms of structure thereof. The jelly-roll type electrode assembly may be fabricated by applying an electrode active material to a metal foil used as a current collector, drying and pressing the same, cutting off the pressed material into a band form having desired width and length, isolating an anode and a cathode using a separator, and winding the band form product in a spiral form. Although the jelly-roll type electrode assembly is suited to formation of a cylindrical battery, it has disadvantages such as detachment of an electrode active material, low space utility, etc. in application to an angular or pouch type battery. On the other hand, a stack type electrode assembly comprises a structure of sequentially stacking plural cathode and anode units, and has an advantage of easily forming an angular battery. However, there are incurred problems such as complicated processes in fabricating the assembly and short circuit occurring due to thrust of an electrode when external impact is applied thereto.
In order to overcome the foregoing problems, an electrode assembly having an improved structure, which is a combination of the jelly-roll type and the stack type assemblies described above, has been developed and called a ‘stack/folding type electrode assembly’ wherein a full cell having a predetermined unit size of a cathode/separator/anode structure or a bi-cell having a predetermined unit size of a cathode (anode)/separator/anode (cathode)/separator/cathode (anode) structure is folded using a continuous separator film having a long length and has been described in detail in published documents, for example, in Korean Laid-Open Patent Publications Nos. 2001-0082058, 2001-0082059 and 2001-0082060, the disclosures of which are incorporated herein by reference in their entireties. Herein, the electrode assembly having such a structure as described above may be referred to as a stack/folding type electrode assembly.
A secondary battery having a structure of a stack type or stack/folding type electrode assembly mounted in the battery case may have different shapes, and a representative example thereof may be a lithium ion polymer battery (LiPB) using a pouch type case made of an aluminum laminate sheet.
The lithium ion polymer battery (LiPB) may have a structure comprising an electrode assembly impregnated with electrolyte wherein the assembly consists of electrodes (cathode and anode) and a separator thermally fused to each other, and a stack type or stack/folding type electrode assembly sealed in a pouch type case formed of an aluminum laminate sheet is mostly used. Accordingly, the lithium ion polymer battery may often be called a ‘pouch type battery’
As described above, in order to fabricate a stack type or stack/folding type electrode assembly, respective unit cells (full cells or bi-cells) should be aligned and wound over a separate film. If the unit cells are even slightly misaligned when the unit cells are laminated at a predetermined interval, a failure may occur during testing after completing the manufacture of a battery. Or, a problem of voltage decrease in charging-discharging after testing the battery may be caused.
Accordingly, there is still a strong need for techniques to solve conventional problems described above.